In blood testing, it is of crucial importance to define an accurate volume of a blood sample, since such accurately defined volume is later on used for certain tests. The accurately defined volume of blood sample is normally diluted by an accurately defined volume of a diluent or a lysing agent, in order to obtain a dilution of typically 1:100 to 1:80000. When counting white blood cells is concerned the dilution is typically 1:400 and when counting red blood cells (RBC) is concerned the dilution is typically 1:40000, in the latter case the dilution normally taking place in two steps. It is obvious, that measurement of sample volumes and dilution liquid volumes must be performed in an accurate and repeatable way such that a correct degree of dilution can always be guaranteed. Apparently, accurate measurement of sample volumes is a critical step in the dilution procedure, since the volumes concerned are extremely small compared to the corresponding diluent volumes.
To be able to obtain an accurate volume of a blood sample it is of crucial importance that the means in which the blood sample is contained is filled in a very precise and accurate and repeatable way.
A sample volume defining device of the above-identified type is known from the co-pending Swedish Patent Application No. 0303157-2. It has usually the form of a disposable cassette.
One problem when filling small cavities or voids or the like for obtaining the accurate and repeatable volume of a liquid sample, preferably a blood sample, is that the cavities are not filled in a proper way due to the formation of air bubbles in the cavities designed for the blood sample. Thus, there will be a difference in the volume between different blood samples and this will have a large significance when later counting the blood cells.
Another problem with known apparatus for testing of small volumes of a liquid sample/blood sample is to see when the apparatus is correctly filled.
Another problem is that the filling of known apparatus for testing of small volumes of a liquid sample/blood is dependent upon the direction of the apparatus when it is filled.